Whole wafer MEMS release process

ABSTRACT

A process for manufacturing a wafer having a multiplicity of MEMS devices such as mirrors with gimbals formed thereon is disclosed. The devices on the wafer include features defined by a wide line between features which extend completely through the wafer, and have a ratio of greater than about 4:1 with respect to the narrow lines which separate individual devices. Each individual device is separated by narrow gaps or line widths which are, for example, about 10 μm. Thus, the etching process is controlled such that the features defined by the wide lines are etched completely through, whereas the individual devices are separated by narrow lines which are not etched completely through the wafer. Therefore, the multiplicity of devices remain attached together even after the wafer is released from a backing wafer. Thus, the wafer with the many devices still attached together allows further processing such as packaging, testing, transport, etc. without the required handling of individual devices.

[0001] This patent claims the benefit of U.S. Provisional PatentApplication No. 60/342,248, filed Dec. 21, 2001, which is incorporatedherein by reference.

FIELD OF THE INVENTION

[0002] This unit relates generally to apparatus and methods formanufacturing MEMS (micro-electromechanical systems) by forming amultiplicity of such devices on a silicon wafer. More specifically, theinvention relates to such a manufacturing process which allows furtherprocessing and/or testing before each individual device is separatedfrom the silicon wafer.

BACKGROUND OF THE INVENTION

[0003] Texas Instruments presently manufactures a two-axis analogmicromirror MEMS device fabricated out of a single piece of material(such as silicon, for example) typically having a thickness of about 115μm. The die layout consists of an oval micromirror, normally 3.8 mm×3.2mm supported on a gimbal frame by two silicon torsional hinges. Thegimbal frame is attached to the die frame by another orthogonal set oftorsional hinges. The micromirror die (i.e. each individual device) isfabricated by etching the 115 μm thick silicon wafer in a specializedICP (Inductively Coupled Plasma) plasma reactor.

[0004] MEMS devices are becoming more and more available and common.However, these devices are extremely small compared to regular machines,but still very large when compared to the individual circuits orcomponents and features found on IC's and other electronic chips. SomeMEMS devices such as the digital micromirror device arrays produced byTexas Instruments are made significantly smaller than most other typesof MEMS devices, but are also very large compared to components on an ICor other chips and use existing geometry and patterning techniquescommon for the productions of semiconductor circuits. For example, smallMEMS devices such as gimbal supported mirror 32 shown in FIG. 2D usedfor optical switching of transmitted data streams are presently on theorder of 3.2×3.8 mm, whereas the mirrors on micromirror arrays used fordisplay devices are typically between about 15-20 microns on a side.Thus, it is seen that MEMS devices are not comfortably compared witheither full-size machines or devices (they are much smaller) or a truearray of micro devices such as IC's, memory chips, and the like (theyare much larger).

[0005] The present invention relates to individual mirror devices formedon a wafer using processing steps some of which have similarity to stepsused in manufacturing IC's and other semiconductor devices.

SUMMARY OF THE INVENTION

[0006] The present invention provides a process for manufacturing aplurality of MEMS devices on a first layer of material, such as forexample, a thin wafer of silicon typically having a thickness of aboutof 115 μm. The process comprises attaching the thin silicon wafer to acarrier or backing wafer and then defining features for each individualdevice of said plurality of devices with a first line width. Theboundary or separation lines between the individual ones of theplurality of devices are defined with a second line width that has athickness less than the thickness of the first line width used to definethe device features.

[0007] After placing both the lines which define the features of theindividual devices and the boundary or separation lines betweenindividual devices, the wafer while attached to the backing wafer isetched such that the lines which define the features of the device areetched through the selected thickness. However, the etching is stoppedbefore the thinner lines which define boundaries of the individualdevices are etched through the thickness of the wafer. This is possiblebecause of the phenomenon called microloading. Microloading is thedifferential etch rate between wide lines and narrow lines (wide linesetch faster) in a plasma reactor. Thus, it is seen that the individualdevices are formed because of the fast etch rate of the wide lines,while at the same time all of the devices on the wafer remain attachedtogether because of the slower etch rate of the thin separation line.The wafer with the devices still attached together is then separatedfrom the backing layer. It should also be noted that the wafer with thedevices could be silicon or another suitable material. Further, thewafer may also undergo other processes before the device is etched. Forexample, electronics, sensors or other mechanical features can becreated by standard IC or MEMS fabrication before the process step ofetching through the wafer is accomplished.

[0008] Therefore, according to embodiments of the present invention, thesilicon wafer with all of the attached devices etched therein can thenbe further processed. For example, further processing may comprisetesting of the torsional gimbals of the individual mirrors by moving themirrors by either soft directed currents of air or spring pins. This isa much faster process than having to handle and test the gimbals on eachseparated mirror. In addition, it is also possible to better clean theattached mirror on the wafer after it has been released from its backinglayer than it is to handle each individual device.

BRIEF DESCRIPTION OF THE DRAWINGS

[0009] The above-mentioned features as well as other features of thepresent invention will be more clearly understood from consideration ofthe following description in connection with the accompanying drawingsin which:

[0010]FIGS. 1A through 1H illustrate the various steps of amanufacturing process;

[0011]FIG. 2A illustrates a mirror wafer;

[0012]FIG. 2B shows a top view and a side view of the fixture forcatching the individual mirror devices upon release;

[0013]FIG. 2C is a cross-section of the apparatus used to release thesilicon device from the backing wafer used by a manufacturing process;

[0014]FIG. 2D is an enlarged view of a pocket on the fixture of FIG. 2Band also shows an individual mirror device caught by the fixture of FIG.2B;

[0015]FIGS. 3A and 3B illustrate two methods of removing wafer wasteareas greater than about 50 μm;

[0016]FIG. 3C illustrates how device features having a separation lessthan a narrow line width may be formed by a first method;

[0017]FIG. 4 illustrates the use of wide separation lines to definedevice features and narrow separation lines to define devices accordingto the present invention;

[0018]FIG. 5 illustrates how device features having a separation lessthan a selected line width may be formed according to the presentinvention;

[0019]FIGS. 6A through 6C illustrates a method of manually separatingthe device wafer from the backing wafer according to the presentinvention; and

[0020]FIG. 7 illustrates a method of separating the individual deviceson a wafer.

DESCRIPTION OF PREFERRED EMBODIMENTS

[0021] The process flow of one method of manufacturing two-axis analogmicromirror MEMS devices wherein the individual dies, elements ordevices are diced or separated by the same through the wafer etch thatforms the features of the mirror is disclosed in FIGS. 1A-1H. As shownin FIG. 1A, a 115 μm thick wafer 10 is bonded to a carrier or backingwafer 12 (see FIG. 1B). Optional alignment marks 14 may then be etchedinto the thin wafer material or other suitable material using a resistlayer 16 along with photolithography and plasma etching as shown in FIG.1C. After the plasma etch, the resist 16 used to form the optionalalignment marks 14 is then stripped as shown in FIG. 1D. The features ofthe micromirror or MEMS devices are then patterned with photolithographyas indicated by line gaps 18 and 20 patterned in a second resist layer22 as is well known by those skilled in the art. This arrangement isshown in FIG. 1E. As shown in FIG. 1F, the mirror features formed by gapor line pattern 18 and 20 are then etched completely through the wafer10 as indicated by reference numbers 24 and 26 using a special ICPplasma reactor and the Bosch process U.S. Pat. No. 5,498,312. It isimportant to note at this point that according to this method ofmanufacturing, at the same time the mirror features such as were etchedcompletely through the wafer 10, the line patterns or etches such asindicated at etch 26 used to separate the individual dies or mirrors asindicated at etch line 24 are also etched completely through the wafer10. After the etching process, the second photo resist layer 22 isstripped away, and the wafer still bonded to the backing wafer is givena gold coat 28 such as shown in FIG. 1G. Finally, the mirror die orindividual mirrors are released from the carrier wafer 12 as shown inFIG. 1H. This is accomplished by placing the combination carrier orbacking wafer 12 and the wafer 10 in a solvent bath to dissolve theagent bonding the carrier wafer 12 and wafer 10 together. The bondingagent is typically a photo resist. Therefore, according to oneembodiment, the solvent for separating the backing wafer 12 from thewafer 10 is acetone.

[0022] Referring now to FIGS. 2A, 2B, 2C and 2D, there is shown thewafer 10 with the individual mirrors etched therein, a fixture forcatching the individual dies or mirrors after they are released from thebacking or carrier wafer 12 (top and side view shown in FIG. 2B), and across-sectional view of the solvent bath with the wafer 10 and fixtureof 2B in place as used during the release process (FIG. 2C). As shown inFIG. 2A, the embodiment illustrates 178 individual mirrors or diesetched into the wafer 10. Likewise, the fixture of 2B shows an equalnumber or 178 pockets such as pocket 30 more clearly seen in the brokenout blown up illustration of FIG. 2D, which catch the individual mirrorsor dies, such as mirror and gimbal structure 32, after they arereleased. The mirror wafer 10 is aligned on the fixture of FIG. 2B sothat each individual mirror is over a pocket 30 that catches the mirrorafter release. As shown in FIG. 2C, the bonded wafer 10 is loaded upsidedown in the fixture so that gravity will pull the individual mirrorsdown into an aligned pocket as they are released from the carrier wafer12.

[0023] This process requires non-standard semiconductor practices andconsequently experiences some problems that may reduce yield. Forexample, each individual die or mirror can have residue on the dieresulting from the release process; (2) each of the die can get dryingspots where they land on the released fixture; (3) some breakout piecesof the original wafer 10 (to be discussed hereinafter) can get stuck tothe mirror die; and (4) some of the die or individual mirrors 32 simplynever get released from the carrier wafer 12 or they get re-stuck to thecarrier wafer 12 when the acetone or alcohol used in a subsequent rinsedries (due to capillary forces). Furthermore, as mentioned, this processis also different from standard semiconductor assembly practices becauseit is very difficult to ship the individual dies that have been releasedfrom the carrier wafer since they break rather easily during routinehandling. Also, there is no way other than an optical inspection of eachindividual die or mirror to identify the known good mirrors. However,optical inspection of such small items is extremely difficult andexpensive. There are also no mechanical or electrical tests that can beperformed on the individual mirrors or dies while they are still bondedto the backing layer to verify whether the mirrors are good or faulty.

[0024] Consequently, since it is very difficult to ship, (if shipping isto occur) the individual dies because they are fragile and cannot beshipped using the accepted methods for shipping electronic die, such asgel-track trays or chip trays. Therefore according to this process, thebonded combination wafer 10 and backing wafer 12 must be shipped. Thus,the release process must also be transferred to the assembly vendor.This means that there may be no yield data available on the mirror diesuntil final testing of the assembled micromirrors and may result in aninability to determine the cause of defaults or the particular processsteps or areas where the defaults occur.

[0025] The present invention relates to individual mirror devices formedon a wafer using processing steps some of which have similarity to stepsused in manufacturing IC's and other semiconductor devices. Referringagain to the process discussed with respect to FIGS. 1A through 1H, itis noted that the described process follows “mask” guidelines whichrequired all features on the wafer or each individual device to becreated by etching trenches, for example, having a 10 μm width. Thisrule or guideline was typically included or followed because of“microloading” which occurs with plasma etching. As discussed above,microloading results because lines of different widths etch at differentrates, and more specifically, “wide” lines etch at a faster rate than“thin” lines. Thus, to provide consistency in etching of features, astandard rule is that all lines including features and separating linesare to be etched by lines 10 μm in width. Consequently, as shown inFIGS. 3A and 3B, if an area 34 (FIG. 3B), that is larger than 10 μm isto be removed, the process discussed above with respect to FIGS. 1Athrough 1h required etching 10 μm lines 37 around the area to be removedso as to leave a break-away area or piece 36 as shown in FIG. 3A. Forexample in FIGS. 3A and 3B, the area to be removed is 50 μm. Thisbreak-away piece or area 36 is then removed after the etching releaseprocess. The break-away area or piece 36 will typically simply fall awayafter the individual dies or mirrors are removed from the backing wafer12. A potential problem with this process is that sometimes thebreak-away areas or pieces 36 are not removed, but instead, stick to oneof the mirror devices and cause a failure. FIG. 3C illustrates thegimbal support structure 38 and a mirror 40 attached to the gimbalsupport structure 38 by a torsion hinge 42, as well as a blow up view ofan alignment stop 44 (there may be more than one) between structure 38and mirror 40 as formed by this process. This illustration shows how alletch lines may be limited to a minimum of 10 μm, yet some parts of thestructure may be divided by a spacing less than 10 μm.

[0026] Other difficulties or problems with the above discussed methodare when the layer 10 with the individual devices was released from thebacking wafer 12.

[0027] The process of this invention uses the differences in“microloading” or in etch rates of wide lines and narrow linesadvantageously. For example, the process of the present invention mayfollow the method discussed above with respect to FIGS. 1A through 1Hfrom FIG. 1A through FIG. 1D. However, as shown in FIG. 4, according tothe present invention, narrow lines 46 are used as the dividing orseparation lines between individual dies (devices such as the mirrordevice 48 and 50), whereas “wide” etching lines such as lines 52 in theillustrated embodiment have a ratio of greater than 4:1 with respect tothe narrow lines 46 and are used to define features of an individualdevice formed on the 115 μm wafer 10. As examples only, the individualdevices are separated by lines having a width of 10 μm, and the featuresin FIGS. 4, 5 and 7 are shown as being defined by lines equal to orgreater than 50 μm. For example, in the embodiment shown in FIG. 4, line52 separates mirror 54 from gimbal structure 56. Therefore, as alsoillustrated, the features on individual devices formed by the fastetching 50 μm lines (such as line 52) are completely etched through the115 μm wafer 10 before the slower etching 10 μm separation lines 46 areable to etch through the wafer. FIG. 5 shows formation of the torsionalhinge or support 58 and an alignment stop 60 using 50 μm lines forseparating features as formed by the process of the present invention.It will be appreciated by those skilled in the art that although siliconis often preferred for such processes, other suitable materials such as,but not limited to, gallium arsenide, quartz and silicon carbide mayalso be used.

[0028] Therefore, by stopping the etching process after the completeetching of the wide lines (e.g. line 52), but before the narrow lines(e.g. line 46) can etch through the wafer, all of the individual devices(or according to the embodiment discussed above the individual 178mirrors and their gimbal structure) are all still attached to eachother. This allows the multiplicity of devices etched into the 115 μmwafer 10 to be removed from the backing wafer 12 still in the shape of awafer or as a single unit. Since all of the individual devices or “dies”are still attached to each other, they are all more likely to separatefrom the backing wafer 12 than was the case using the method discussedabove with respect to FIGS. 1A-1H and 2A-2C. Thus, the yield willincrease.

[0029] A process for releasing wafer 10 from its backing wafer 12 andthen cleaning the released “etched” wafer 10 is illustrated anddiscussed with respect to FIGS. 6A through 6C. For example, as shown inFIG. 6C, the wafer combination 62 consisting of etched wafer 10 andbacking wafer 12 is soaked in acetone 64 for a selected period of timeto substantially dissolve the adhesive (for example, resist) which bondsthe wafer 10 to backing wafer 12. Then as shown in FIG. 6B, an edge ofthe wafer 10 with the individual devices etched therein is then grippedsuch as by tweezers 66 and slid or pulled off of the carrier wafer 12 asshown in FIG. 6C. This process can also be done by automated tooling.The removed wafer 10 is then preferably soaked in a fresh bath of cleanacetone for about five minutes to remove any residue so as to avoidspots on the devices. The micromirror wafer should quickly be placed inthe fresh bath to assure that the wafer stays wet with acetone. Afterthe wafer has been soaked in the fresh acetone bath, the wafer ispreferably rinsed in a hot IPA bath for about five minutes. The wafer isthen removed from the hot IPA bath. The removal of the wafer 10 from thehot IPA bath may be a slow process so that the IPA sheets off of thewafer or alternately, the wafer may be dried using an IPA vapor dryer.

[0030] As was disclosed above, it is extremely difficult to test theindividual mirrors after they have been separated from each otheraccording to the process discussed with respect to FIGS. 1A-1H. However,it is now possible to carefully clamp the etched wafer 10 with all 178mirrors and gimbals in a fixture and then test the individual devices ormirrors to determine defects by applying a slight force. For example, aspring pin or air pressure may be used to verify proper movement of themirrors. Subsequent to testing, the individual devices or gimbal mirrora structure on the wafers such as structures 48 and 50 can then beseparated from each other by using a punch 68 and anvil 70 to crack theconnecting material 72 which remains in the area of the 10 μm lines 46after etching. This is shown in FIG. 7.

[0031] While this invention has been described with reference toillustrative embodiments, this description is not intended to beconstrued in a limiting sense. Various modifications and combinations ofthe illustrative embodiments, as well as other embodiments of theinvention, will be apparent to persons skilled in the art upon referenceto the description. It is therefore intended that the appended claimsencompass any such modifications or embodiments.

I claim:
 1. A process for manufacturing a plurality of MEMS devices on afirst layer of material of a selected thickness comprising: attachingsaid first layer of material to a backing layer of material; definingfeatures on each individual ones of said plurality of MEMS devices withfirst lines having at least a first selected width; defining boundarylines between individual ones of said plurality of MEMS devices withsecond lines having a width that is less than said first selected width;simultaneously etching said first lines and said second lines until saidfirst lines defining device features are etched through said selectedthickness; stopping said etching before said second lines definingboundaries are etched through said first selected thickness; andseparating said first layer with said plurality of devices attachedtogether from said backing layer.
 2. The process of claim 1 andcomprising further processing of said separated first layer.
 3. Theprocess of claim 2 when said further processing comprising testing saiddevices while still attached together on said first layer.
 4. Theprocess of claim 2 wherein said further processing comprises separatingeach individual device of said first layer from each other.
 5. Theprocess of claim 1 wherein said further processing comprises cleaningsaid devices while still attached together subsequent to said separationstep.
 6. The process of claim 1 and further comprising packing saidseparated wafer with said devices still attached together for storage orshipping.
 7. The process of claim 1 wherein said first width of saidfirst lines have a ratio greater than 4:1 with respect to said width ofsaid second lines.
 8. The process of claim 1 wherein said first selectedwidth is at least about 50 μm and said second width is about 10 μm. 9.The process of claim 1 wherein said first layer of material is selectedfrom the group consisting of silicon, gallium arsenide, quartz andsilicon carbide.
 10. The process of claim 9 wherein said first layer ofmaterial is silicon.
 11. A process for manufacturing a plurality ofgimbal mirror devices on a first layer of material of a selectedthickness comprising: attaching said first layer of material to abacking layer of material; defining features on each individual ones ofsaid plurality of gimbal mirror devices with first lines having at leasta first selected width; defining boundary lines between individual onesof said plurality of gimbal mirror devices with second lines having awidth that is less than said first selected width; simultaneouslyetching said first lines and said second lines until said first linesdefining gimbal mirror features are etched through said first selectedthickness; stopping said etching before said second lines definingboundaries are etched through said first selected thickness; andseparating said first layer with said plurality of gimbal mirror devicesattached together from said backing layer.
 12. The process of claim 11further comprising testing individual devices defined on said firstlayer.
 13. The process of claim 11 further comprising separating eachindividual gimbal mirror device from said first layer.
 14. The processof claim 11 wherein said further processing comprises cleaning saidgimbal mirror while still attached together subsequent to said step ofseparating said first layer from said backing layer.
 15. The process ofclaim 11 and further comprising packing said separated wafer with saiddevices still attached together for storage or shipping.
 16. The processof claim 11 wherein said first lines have a width at least equal toabout 50 μm and said second lines have a width of about 10 μm.
 17. Theprocess of claim 11 wherein said first layer is a silicon wafer.
 18. Awafer defining a plurality of MEMS devices attached together comprising:at least two features of said MEMS devices separated by first linesetched completely through said wafer, said first line having at least afirst selected width; second lines etched part way through said waferdefining individual ones of said plurality of MEMS devices, said secondlines having a second width which is less than said first selectedwidth.
 19. The wafer of claim 18 wherein said width of said first lineshave a ratio greater than 4:1 with respect to said width of said secondlines.
 20. The wafer of claim 18 wherein said first selected width is atleast about 50 μm and said second width is about 10 μm.
 21. A waferdefining a plurality of gimbal mirror devices attached togethercomprising: at least two features of said gimbal mirror devicesseparated by first lines etched completely through said wafer, saidfirst lines having at least a first selected width; second lines etchedpart way through said wafer defining individual ones of said pluralityof gimbal mirror devices, said second lines having a second width whichis less than said first selected width.